PROJECT ABSTRACT Osteochondral grafts or Matrix-Assisted-Cell-Implantation (MACI) are promising strategies to treat chondral lesions as articular cartilage does not heal spontaneously. One characteristic shared by intrinsic reparative processes and surgical therapies is an apparent lack of lateral integration of repair or graft tissue with the host cartilage that can lead to poor prognosis. The goal of this R03 project is to demonstrate the feasibility of employing a relatively non-invasive adjunct such as continuous low-intensity ultrasound (cLIUS) to enhance integration. Previous therapies have had varied levels of success with pulsed LIUS (pLIUS @ 1.5 MHz), but have used an arbitrary treatment that doesn't take into account knee tissue properties or tissue response to LIUS. To progress beyond the status quo that employs an empirically derived pLIUS at 1.5 MHz for in vivo applications, first, our approach employs cLIUS as it couples more energy when compared to pLIUS. Importantly, our ongoing innovative research represents a new and substantive departure from the status quo by shifting the focus to the following: (1) identification of a clinically relevant tissue resonant frequency range of 3.8 0.3 MHz, where cLIUS induced bioeffects are maximized by accounting for the tissue properties, and (2) understanding of the propagation of cLIUS at these higher frequencies in the joint space and in the vicinity of the defect to permit optimal cLIUS regimens and route of delivery. Taken together, these innovations enable us to deliver therapeutic dosages of cLIUS to enable cartilage repair. Based on our promising in vitro findings that demonstrate the cLIUS to be chondroprotective and chondrpoinductive, our central hypothesis is that cLIUS will aid in the cartilage-to- cartilage integration of critical sized defects in a rabbit model of subchondral injury by promoting migration and enhancing chondrogenesis to a degree greater than approaches without cLIUS treatment. To test this, one specific aim will be undertaken: 1: Demonstrate enhanced cartilage integration under LIUS. Our objective is to demonstrate that cLIUS will achieve reproducible integrative cartilage repair and maintain normal articular cartilage histologic and biomechanical characteristics in a rabbit model of chondral repair. Rabbits will be euthanized after 4 and 12 weeks of cLIUS treatment, at which time cartilage regeneration and integration will be assessed. In addition to non-stimulated controls, group comparison will be made with the standard US regimen that uses pulsed US signal at 1.5 MHz with 1 kHz repeat. Successful completion of this work is expected to lead to the development of a cLIUS-based regimen and delivery system capable of promoting integration at the graft interface. This presents an exciting potential treatment modality for orthopedic surgeons attempting to treat patients with this difficult clinical problem. This would, in turn, directly address the needs of 46 million Americans who suffer from osteoarthritis at an estimated cost of $128 billion annually in the U.S. alone.